1. Field of the Invention
This invention relates generally to wireless communications systems, and more particularly, to a method of controlling alignment of an optical wireless communication link between an optical transmitting station (transceiver) and an optical receiving station (transceiver) which are not capable of sensing positional information of the remote station's beam relative to the local station's detector.
2. Description of the Prior Art
An optical wireless link system comprises at least two stations, each of which contains an optical transmitter (or transceiver) and an optical receiver (or transceiver). The transmitter is able to change the direction of its transmitted beam by known amounts of angular displacement. The receiver generally sees this motion and sends position correction information back to the transmitter.
In some instances however, the receiver is not equipped with any position sensing capability to detect the positioning accuracy of the incident beam relative to the receiving station detector. In such systems, active tracking between stations is not typically employed; rather, the stations will switch to a “hold” mode, where the direction of the beam is held constant once a link is established. In order to establish the link (a procedure termed “acquisition”), such architectures must rely solely on binary feedback in which a sample was received or it was not received (either the beam is aligned with the receiving station data detector or it is not so aligned). This is problematic since some feedback may be detected by the receiving station as soon as the beam is close to the receiving station's data detector (i.e. locking onto the “halo” or edge of the laser). Ideally, the beam should be centered on the detector since the center is brighter (higher SNR), and the system will be more immune to slight vibrations. Once the foregoing feedback is detected, some method must be used to determine if further adjustment is necessary, or if the alignment is sufficient for data communication. In one method, the acquisition spiral may be continued until the alignment is good enough to meet some predefined criteria (e.g. 100 samples in a row). If the alignment is slightly off center, however, it will likely be incapable of transferring data at a high rate due to insufficient alignment. The poor data rate may trigger the need for a “re-acquisition”. Such techniques may then switch back and forth between the acquisition and hold modes, without ever establishing a good data link.
In view of the foregoing, it would be desirable and advantageous in the optical wireless communication art to provide a technique that allows an optical wireless communication link to be used reliably without the need for position sensing capabilities.